Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

The objective of this work is to analyse the density structure of the continental forearc in the northern segment of the 1960 Mw 9.6 Valdivia earthquake. Regional 2D and local 3D density models have been obtained from available gravity data in the area, complemented by new gravimetric stations. Models are constrained by independent geophysical and geological information and new TEM and MT soundings. The results show a segmentation of the continental wedge along and perpendicular to the margin, highlighting a high-density anomaly, below the onshore forearc basin, that limits the late Paleozoic–early Mesozoic metamorphic basement in the region where Chaitenia terrane has been proposed. A progressive landward shift of this anomaly correlates with the high slip patch of the giant 1960 Mw 9.6 Valdivia earthquake. Based on these results, we propose that the horizontal extension of the less rigid basement units conforming the marine wedge and Coastal Cordillera domain could modify the process of stress loading during the interseismic periods, and also that changes in position and extension of the late Paleozoic–early Mesozoic accretionary complex could be linked with the frictional properties of the interplate boundary. This analysis provides new evidence of the role of the overriding plate structure in the seismotectonic process in subduction zones.

The role of the pre-Alpine polycrystalline basement in the paleogeographic configuration of multiple Neotethyan oceanic basins

The study provides a deeper understanding of the early Mesozoic paleogeogeographic spatial-temporal relationship by studying the two Adria-Europe intervening basement blocks. The Drina-Ivanjica and Pelagonian crustal fragments play important role in the internal early Alpine oceanic constitution further controlling the late Jurassic emplacement of Tethyan Dinaric-Hellenic ophiolites. The proposed paleogeographic reassessment is driven by the new paleocontinental inheritance data associated with the Variscan – pre-Variscan basement terranes. The recently published data suggest an Avalonian-type inheritance of the Pelagonian basement block which indicates a different pre-Variscan plate-tectonic journey, including separate spatial arrangement during Variscan amalgamation. In turn, Cadomian-type basement inheritance has been documented within the sliced Adria microplate. Thus, the Avalonian inheritance place the Pelagonian block away from the Apulia/Adria (Dinarides). In the investigated context of Paleozoic-Mesozoic paleogeographic transition, the Pelagonian block may represent a segment of the Cimmerian ribbon continent or southernmost segment of the Variscan Europe. With regards the nearby Adria microplate, a Triassic-Jurassic oceanic opening led to the decoupling (spreading away from the main Adria microplate) of the Drina-Ivanjica block. The rifting is in line with the simultaneous yet opposite or westward-directed drift of the Pelagonides. The breakup of south European Variscan configuration eventually result in the spatial alignment of the two basement fragments referred to as the “Drina–Pelagonide continental splinter”. By linking the paleogeographic pre-Jurassic–Jurassic relationship between these continental units, the two landlocked Neotethyan Vardar s.l. basins are extrapolated, “Dinaric Tethys” / Inner Dinaric-(Mirdita-Pindos) and the main Vardar Ocean (Western Vardar Zone).

Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.